The present invention relates to a speed control apparatus for a sewing machine capable of operating at at least two speeds.
In a conventional speed control apparatus for a sewing machine, a voltage having a magnitude determined by the amount of depression of a pedal of the sewing machine is converted to a pulse signal (hereinafter referred to as an FG signal) having a pulse frequency related to the rotational speed of the sewing machine is produced, the number of the pulses of the FG signal is counted during periods between PG signal pulses and the current flowing through a clutch coil functioning as a driving mechanism or through a brake coil functioning as a brake mechanism is actuated when the counted value exceeds a predetermined value. The conventional circuit used for producing the FG signal has two input terminals, and the frequency of the FG signal is determined by the external inputs applied to the two input terminals. One of the two input terminals receives the voltage corresponding to the amount of depression of the pedal, and the other a voltage which effects driving of the sewing machine at a predetermined speed and which is independent of the voltage applied to the first input terminal. The two input terminals are hereinafter referred to as terminal VC and terminal L respectively.
In the conventional apparatus, two currents I.sub.a and I.sub.b are produced in correspondence to the voltages applied to the terminals VC and L, a capacitor is charged by the currents I.sub.a and I.sub.b, and the FG signal is produced in response to the charging and discharging of the capacitor. However, since the capacitor is connected in common to the terminals VC and L, it is necessary to adjust the frequency of the FG signal by providing two variable resistors, one for adjusting the current I.sub.a generated in response to the input on the terminal VC and the other for adjusting the current I.sub.b generated in response to the input on the terminal L.
However, in the conventional apparatus, the capacitor is charged by both currents I.sub.a and I.sub.b when an active signal is applied to the input terminal VC, while only the current I.sub.b is supplied to the capacitor to charge it when an active signal is applied to the terminal L. Therefore, when the current I.sub.b is adjusted by the variable resistor as mentioned above, the sum of the currents I.sub.a and I.sub.b applied to the capacitor when the terminal VC receives an input is undesirably changed, thereby changing the output frequency. Under these circumstances, it is necessary to readjust the current I.sub.b with the other variable resistor. As a result, it is impossible in the conventional apparatus to adjust the output frequency of the FG signal independently with both variable resistors.
Further, the conventional apparatus has disadvantage that the current for charging the capacitor is the sum of the currents I.sub.a and I.sub.b. Thus, the capacitor is usually charged by a current above the level of the current I.sub.b, which results in the speed set by the variable resistor for adjusting the current I.sub.a usually being higher than the speed set by the variable resistor for adjusting the current I.sub.b.